1. Field of the Invention
The present invention relates to a hand position detecting device which detects rotational positions of seconds, center and hour hands.
2. Description of the Related Art
A conventional hand position detecting device used for a hand type timepiece is disclosed in Japanese Patent No. 3872688.
The hand position detecting device comprises a first drive system in which a first drive motor transmits its rotation to a seconds wheel which in turn causes a seconds hand to sweep around a dial, a second drive system in which a second drive motor transmits its rotations to the center and hour wheels to cause the center and hour hands, respectively, to sweep around the dial. The hand position detecting device also comprises a photosensor including a light emission element and a photo detection element. The photosensor optically detects a first, a second and a third light-passing apertures provided respectively in the seconds, center and hour wheels with the aid of the light emission element and the photo detection element when the seconds, center and hour wheels of the first and second drive systems are rotated after pointing to the same direction on the same axis. The hand position detecting device detects respective rotational positions of the seconds, center and hour wheels based on detected signals from the photosensor and hence rotational positions of the seconds, center and hour hands are determined.
According to the conventional hand position detecting device, the rotational positions of the hands are detected in response to a signal which is output to the photosensor in synchronization with timing to output a pulse to one of winding start and winding end of each coil in the first and second drive motors. The photosensor detects the rotational positions at every two steps of the first and second drive motors. Therefore, the rotational positions may not be detected correctly in the case where the first and second drive motors do not operate normally due to an external magnetic field.